The search for new therapeutic agents has been greatly aided in recent years by a better understanding of the structure of enzymes and other biomolecules associated with diseases. One important class of enzymes that has been the subject of extensive study is protein kinases.
Protein kinases constitute a large family of structurally related enzymes that are responsible for the control of a variety of signal transduction processes within the cell. (See, Hardie, G. and Hanks, S. The Protein Kinase Facts Book, I and II, Academic Press, San Diego, Calif.: 1995). Protein kinases are thought to have evolved from a common ancestral gene due to the conservation of their structure and catalytic function. Almost all kinases contain a similar 250-300 amino acid catalytic domain. The kinases may be categorized into families by the substrates they phosphorylate (e.g., protein-tyrosine, protein-serine/threonine, lipids, etc.). Sequence motifs have been identified that generally correspond to each of these kinase families (See, for example, Hanks, S. K., Hunter, T., FASEB J. 1995, 9, 576-596; Knighton et al., Science 1991, 253, 407-414; Hiles et al., Cell 1992, 70, 419-429; Kunz et al., Cell 1993, 73, 585-596; Garcia-Bustos et al., EMBO J. 1994, 13, 2352-2361).
In general, protein kinases mediate intracellular signaling by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. These phosphorylation events act as molecular on/off switches that can modulate or regulate the target protein biological function. These phosphorylation events are ultimately triggered in response to a variety of extracellular and other stimuli. Examples of such stimuli include environmental and chemical stress signals (e.g., osmotic shock, heat shock, ultraviolet radiation, bacterial endotoxin, and H2O2), cytokines (e.g., interleukin-1 (IL-1) and tumor necrosis factor a (TNF-α)), and growth factors (e.g., granulocyte macrophage-colony-stimulating factor (GM-CSF), and fibroblast growth factor (FGF)). An extracellular stimulus may affect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis, and regulation of the cell cycle.
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events as described above. These diseases include, but are not limited to, autoimmune diseases, inflammatory diseases, bone diseases, metabolic diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease, and hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
The Aurora family of serine/threonine kinases is essential for cell proliferation [Bischoff, J. R. & Plowman, G. D. (The Aurora/Ipl1p kinase family: regulators of chromosome segregation and cytokinesis) Trends in Cell Biology 9, 454-459 (1999); Giet, R. and Prigent, C. (Aurora/Ipl1p-related kinases, a new oncogenic family of mitotic serine-threonine kinases) Journal of Cell Science 112, 3591-3601 (1999); Nigg, E. A. (Mitotic kinases as regulators of cell division and its checkpoints) Nat. Rev. Mol. Cell Biol. 2, 21-32 (2001); Adams, R. R, Carmena, M., and Earnshaw, W. C. (Chromosomal passengers and the (aurora) ABCs of mitosis) Trends in Cell Biology 11, 49-54 (2001)]. Inhibitors of the Aurora kinase family therefore have the potential to block growth of all tumour types.
The three known mammalian family members, Aurora-A (“2”), B (“1”) and C (“3”), are highly homologous proteins responsible for chromosome segregation, mitotic spindle function and cytokinesis. Aurora expression is low or undetectable in resting cells, with expression and activity peaking during the G2 and mitotic phases in cycling cells. In mammalian cells proposed substrates for Aurora include histone H3, a protein involved in chromosome condensation, and CENP-A, myosin II regulatory light chain, protein phosphatase 1, TPX2, all of which are required forcell division.
Since its discovery in 1997 the mammalian Aurora kinase family has been closely linked to tumorigenesis. The most compelling evidence for this is that over-expression of Aurora-A transforms rodent fibroblasts (Bischoff, J. R., et al. A homologue of Drosophila aurora kinase is oncogenic and amplified in human colorectal cancers. EMBO J. 17, 3052-3065 (1998)). Cells with elevated levels of this kinase contain multiple centrosomes and multipolar spindles, and rapidly become aneuploid. The oncogenic activity of Aurora kinases is likely to be linked to the generation of such genetic instability. Indeed, a correlation between amplification of the aurora-A locus and chromosomal instability in mammary and gastric tumours has been observed. (Miyoshi, Y., Iwao, K., Egawa, C., and Noguchi, S. Association of centrosomal kinase STK15/BTAK mRNA expression with chromosomal instability in human breast cancers. Int. J. Cancer 92, 370-373 (2001). (Sakakura, C. et al. Tumor-amplified kinase BTAK is amplified and overexpressed in gastric cancers with possible involvement in aneuploid formation. British Journal of Cancer 84, 824-831 (2001)). The Aurora kinases have been reported to be over-expressed in a wide range of human tumours. Elevated expression of Aurora-A has been detected in over 50% of colorectal cancers (Bischoff, J. R., et al. A homologue of Drosophila aurora kinase is oncogenic and amplified in human colorectal cancers. EMBO J. 17, 3052-3065 (1998)) (Takahashi, T., et al. Centrosomal kinases, HsAIRk1 and HsAIRK3, are overexpressed in primary colorectal cancers. Jpn. J. Cancer Res. 91, 1007-1014 (2000)), ovarian cancers (Gritsko, T. M. et al. Activation and overexpression of centrosome kinase BTAK/Aurora-A in human ovarian cancer. Clinical Cancer Research 9, 1420-1426 (2003)), and gastric tumors (Sakakura, C. et al. Tumor-amplified kinase BTAK is amplified and overexpressed in gastric cancers with possible involvement in aneuploid formation. British Journal of Cancer 84, 824-831 (2001)), and in 94% of invasive duct adenocarcinomas of the breast (Tanaka, T., et al. Centrosomal kinase AIK1 is overexpressed in invasive ductal carcinoma of the breast. Cancer Research. 59, 2041-2044 (1999)). High levels of Aurora-A have also been reported in renal, cervical, neuroblastoma, melanoma, lymphoma, pancreatic and prostate tumour cell lines. (Bischoff, J. R., et al. A homologue of Drosophila aurora kinase is oncogenic and amplified in human colorectal cancers. EMBO J. 17, 3052-3065 (1998) (Kimura, M., Matsuda, Y., Yoshioka, T., and Okano, Y. Cell cycle-dependent expression and centrosomal localization of a third human Aurora/Ipl1-related protein kinase, AIK3. Journal of Biological Chemistry 274, 7334-7340 (1999)) (Zhou et al. Tumour amplified kinase STK15/BTAK induces centrosome amplification, aneuploidy and transformation Nature Genetics 20: 189-193 (1998))(Li et al. Overexpression of oncogenic STK15/BTAK/Aurora-A kinase in human pancreatic cancer Clin Cancer Res. 9(3):991-7 (2003)). Amplification/overexpression of Aurora-A is observed in human bladder cancers and amplification of Aurora-A is associated with aneuploidy and aggressive clinical behaviour (Sen S. et al. “Amplification/overexpression of a mitotic kinase gene in human bladder cancer” J. Natl. Cancer Inst. 94(17):1320-9 (2002)). Moreover, amplification of the aurora-A locus (20q13) correlates with poor prognosis for patients with node-negative breast cancer (Isola, J. J., et al. “Genetic aberrations detected by comparative genomic hybridization predict outcome in node-negative breast cancer” American Journal of Pathology 147, 905-911 (1995)). Aurora-B is highly expressed in multiple human tumour cell lines, including leukemic cells (Katayama et al. Human AIM-1: cDNA cloning and reduced expression during endomitosis in megakaryocyte-lineage cells. Gene 244:1-7)). Levels of this enzyme increase as a function of Duke's stage in primary colorectal cancers (Katayama, H. et al. Mitotic kinase expression and colorectal cancer progression. Journal of the National Cancer Institute 91, 1160-1162 (1999)). Aurora-C, which is normally only found in germ cells, is also over-expressed in a high percentage of primary colorectal cancers and in a variety of tumour cell lines including cervical adenocarinoma and breast carcinoma cells (Kimura, M., Matsuda, Y., Yoshioka, T., and Okano, Y. Cell cycle-dependent expression and centrosomal localization of a third human Aurora/Ipl1-related protein kinase, AIK3. Journal of Biological Chemistry 274, 7334-7340 (1999). (Takahashi, T., et al. Centrosomal kinases, HsAIRk1 and HsAIRK3, are overexpressed in primary colorectal cancers. Jpn. J. Cancer Res. 91, 1007-1014 (2000)).
Based on the known function of the Aurora kinases, inhibition of their activity should disrupt mitosis leading to cell cycle arrest. In vivo, an Aurora inhibitor therefore slows tumor growth and induces regression.
Elevated levels of all Aurora family members are observed in a wide variety of tumour cell lines. Aurora kinases are over-expressed in many human tumors and this is reported to be associated with chromosomal instability in mammary tumors (Miyoshi et al. 2001 92, 370-373).
Aurora-2 is highly expressed in multiple human tumor cell lines and levels increase as a function of Duke's stage in primary colorectal cancers [Katayama, H. et al. (Mitotic kinase expression and colorectal cancer progression) Journal of the National Cancer Institute 91, 1160-1162 (1999)]. Aurora-2 plays a role in controlling the accurate segregation of chromosomes during mitosis. Misregulation of the cell cycle can lead to cellular proliferation and other abnormalities. In human colon cancer tissue, the Aurora-2 protein has been found to be over-expressed [Bischoff et al., EMBO J., 17, 3052-3065 (1998); Schumacher et al., J. Cell Biol., 143, 1635-1646 (1998); Kimura et al., J. Biol. Chem., 272, 13766-13771 (1997)]. Aurora-2 is over-expressed in the majority of transformed cells. Bischoff et al. found high levels of Aurora-2 in 96% of cell lines derived from lung, colon, renal, melanoma and breast tumors (Bischoff et al. EMBO J. 1998 17, 3052-3065). Two extensive studies show elevated Aurora-2 in 54% and 68% (Bishoff et al. EMBO J. 1998 17, 3052-3065) (Takahashi et al. 2000 Jpn J Cancer Res. 91, 1007-1014) of colorectal tumours and in 94% of invasive duct adenocarcinomas of the breast (Tanaka et al. 1999 59, 2041-2044).
Aurora-1 expression is elevated in cell lines derived from tumors of the colon, breast, lung, melanoma, kidney, ovary, pancreas, CNS, gastric tract and leukemias (Tatsuka et al. 1998 58, 4811-4816).
High levels of Aurora-3 have been detected in several tumour cell lines, although it is restricted to testis in normal tissues (Kimura et al. 1999 274, 7334-7340). Over-expression of Aurora-3 in a high percentage (c. 50%) of colorectal cancers has also been documented (Takahashi et al. 2000 Jpn J Cancer Res. 91, 1007-1014). In contrast, the Aurora family is expressed at a low level in the majority of normal tissues, the exceptions being tissues with a high proportion of dividing cells such as the thymus and testis (Bischoff et al. EMBO J. 1998 17, 3052-3065).
For further review of the role Aurora kinases play in proliferative disorders, see Bischoff, J. R. & Plowman, G. D. (The Aurora/Ipl1p kinase family:regulators of chromosome segregation and cytokinesis) Trends in Cell Biology 9, 454-459 (1999); Giet, R. and Prigent, C. (Aurora/Ipl1p-related kinases, a new oncogenic family of mitotic serine-threonine kinases) Journal of Cell Science 112, 3591-3601 (1999); Nigg, E. A. (Mitotic kinases as regulators of cell division and its checkpoints) Nat. Rev. Mol. Cell Biol. 2, 21-32 (2001); Adams, R. R, Carmena, M., and Earnshaw, W. C. (Chromosomal passengers and the (aurora) ABCs of mitosis) Trends in Cell Biology 11, 49-54 (2001); and Dutertre, S., Descamps, S., & Prigent, P. (On the role of aurora-A in centrosome function) Oncogene 21, 6175-6183 (2002).
Cyclin-dependent kinases (CDKs) are serine/threonine protein kinases consisting of a β-sheet rich amino-terminal lobe and a larger carboxy-terminal lobe that is largely α-helical. The CDKs display the 11 subdomains shared by all protein kinases and range in molecular mass from 33 to 44 kD. This family of kinases, which includes CDK1, CKD2, CDK4, and CDK6, requires phosphorylation at the residue corresponding to CDK2 Thr160 in order to be fully active [Meijer, L., Drug Resistance Updates 2000, 3, 83-88].
Each CDK complex is formed from a regulatory cyclin subunit (e.g., cyclin A, B1, B2, D1, D2, D3, and E) and a catalytic kinase subunit (e.g., CDK1, CDK2, CDK4, CDK5, and CDK6). Each different kinase/cyclin pair functions to regulate the different and specific phases of the cell cycle known as the G1, S, G2, and M phases [Nigg, E., Nature Reviews 2001, 2, 21-32; Flatt, P., Pietenpol, J., Drug Metabolism Reviews 2000, 32, 283-305].
The CDKs have been implicated in cell proliferation disorders, particularly in cancer. Cell proliferation is a result of the direct or indirect deregulation of the cell division cycle and the CDKs play a critical role in the regulation of the various phases of this cycle. For example, the over-expression of cyclin D1 is commonly associated with numerous human cancers including breast, colon, hepatocellular carcinomas and gliomas [Flatt, P., Pietenpol, J., Drug Metabolism Reviews 2000, 32, 283-305]. The CDK2/cyclin E complex plays a key role in the progression from the early G1 to S phases of the cell cycle and the overexpression of cyclin E has been associated with various solid tumors. Therefore, inhibitors of cyclins D1, E, or their associated CDKs are useful targets for cancer therapy [Kaubisch, A., Schwartz, G., The Cancer Journal 2000, 6, 192-212].
CDKs, especially CDK2, also play a role in apoptosis and T-cell development. CDK2 has been identified as a key regulator of thymocyte apoptosis [Williams, O., et al, European Journal of Immunology 2000, 709-713]. Stimulation of CDK2 kinase activity is associated with the progression of apoptosis in thymocytes, in response to specific stimuli. Inhibition of CDK2 kinase activity blocks this apoptosis resulting in the protection of thymocytes.
In addition to regulating the cell cycle and apoptosis, the CDKs are directly involved in the process of transcription. Numerous viruses require CDKs for their replication process. Examples where CDK inhibitors restrain viral replication include human cytomegalovirus, herpes virus, and varicella-zoster virus [Meijer, L., Drug Resistance Updates 2000, 3, 83-88].
Inhibition of CDK is also useful for the treatment of neurodegenerative disorders such as Alzheimer's disease. The appearance of Paired Helical Filaments (PHF), associated with Alzheimer's disease, is caused by the hyperphosphorylation of Tau protein by CDK5/p25 [Meijer, L., Drug Resistance Updates, 2000 3, 83-88].
Glycogen synthase kinase-3 (GSK-3) is a serine/threonine protein kinase comprised of α and β isoforms that are each encoded by distinct genes [Coghlan et al., Chemistry & Biology 2000, 7, 793-803; and Kim and Kimmel, Curr. Opinion Genetics Dev., 2000 10, 508-514]. GSK-3 has been implicated in various diseases including diabetes, Alzheimer's disease, CNS disorders such as manic depressive disorder and neurodegenerative diseases, and cardiomyocyte hypertrophy [PCT Application Nos.: WO 99/65897 and WO 00/38675; and Haq et al., J. Cell Biol. 2000, 151, 117-130]. These diseases are associated with the abnormal operation of certain cell signaling pathways in which GSK-3 plays a role. GSK-3 has been found to phosphorylate and modulate the activity of a number of regulatory proteins. These proteins include glycogen synthase, which is the rate limiting enzyme necessary for glycogen synthesis, the microtubule associated protein Tau, the gene transcription factor β-catenin, the translation initiation factor e1F2B, as well as ATP citrate lyase, axin, heat shock factor-1, c-Jun, c-myc, c-myb, CREB, and CEPBα. These diverse protein targets implicate GSK-3 in many aspects of cellular metabolism, proliferation, differentiation, and development.
In a GSK-3 mediated pathway that is relevant for the treatment of type II diabetes, insulin-induced signaling leads to cellular glucose uptake and glycogen synthesis. Along this pathway, GSK-3 is a negative regulator of the insulin-induced signal. Normally, the presence of insulin causes inhibition of GSK-3 mediated phosphorylation and deactivation of glycogen synthase. The inhibition of GSK-3 leads to increased glycogen synthesis and glucose uptake [Klein et al., PNAS 1996, 93, 8455-8459; Cross et al., Biochem. 1 1994, 303, 21-26); Cohen, Biochem. Soc. Trans. 1993, 21, 555-567; and Massillon et al., Biochem J. 1994, 299, 123-128]. However, in a diabetic patient, where the insulin response is impaired, glycogen synthesis and glucose uptake fail to increase despite the presence of relatively high blood levels of insulin. This leads to abnormally high blood levels of glucose with acute and long-term effects that may ultimately result in cardiovascular disease, renal failure and blindness. In such patients, the normal insulin-induced inhibition of GSK-3 fails to occur. It has also been reported that in patients with type II diabetes, GSK-3 is overexpressed [see, PCT Application: WO 00/38675]. Therapeutic inhibitors of GSK-3 are therefore potentially useful for treating diabetic patients suffering from an impaired response to insulin.
GSK-3 activity is associated with Alzheimer's disease. This disease is characterized by the well-known β-amyloid peptide and the formation of intracellular neurofibrillary tangles. The neurofibrillary tangles contain hyperphosphorylated Tau protein, in which Tau is phosphorylated on abnormal sites. GSK-3 is known to phosphorylate these abnormal sites in cell and animal models. Furthermore, inhibition of GSK-3 has been shown to prevent hyperphosphorylation of Tau in cells [Lovestone et al., Current Biology 1994, 4, 1077-86; and Brownlees et al., Neuroreport 1997, 8, 3251-55]. Therefore, GSK-3 activity promotes generation of the neurofibrillary tangles and the progression of Alzheimer's disease.
Another substrate of GSK-3 is β-catenin, which is degradated after phosphorylation by GSK-3. Reduced levels of β-catenin have been reported in schizophrenic patients and have also been associated with other diseases related to increase in neuronal cell death [Zhong et al., Nature 1998, 395, 698-702; Takashima et al., PNAS 1993, 90, 7789-93; and Pei et al., J. Neuropathol. Exp 1997, 56, 70-78].
GSK-3 activity is associated with stroke [Wang et al., Brain Res 2000, 859, 381-5; Sasaki et al., Neurol Res 2001, 23, 588-92; Hashimoto et al., J. Biol. Chem 2002, 277, 32985-32991].
The Janus kinases (JAK) are a family of tyrosine kinases consisting of JAK1, JAK2, JAK3 and TYK2. The JAKs play a critical role in cytokine signaling. The down-stream substrates of the JAK family of kinases include the signal transducer and activator of transcription (STAT) proteins. JAK/STAT signaling has been implicated in the mediation of many abnormal immune responses such as allergies, asthma, autoimmune diseases such as transplant rejection, rheumatoid arthritis, amyotrophic lateral sclerosis and multiple sclerosis as well as in solid and hematologic malignancies such as leukemias and lymphomas. The pharmaceutical intervention in the JAK/STAT pathway has been reviewed [Frank Mol. Med. 1999, 5, 432-456 and Seidel et al., Oncogene 2000, 19, 2645-2656].
JAK1, JAK2, and TYK2 are ubiquitously expressed, while JAK3 is predominantly expressed in hematopoietic cells. JAK3 binds exclusively to the common cytokine receptor gamma chain (γc) and is activated by IL-2, IL-4, IL-7, IL-9, and IL-15. The proliferation and survival of murine mast cells induced by IL-4 and IL-9 have, in fact, been shown to be dependent on JAK3- and γc-signaling [Suzuki et al., Blood 2000, 96, 2172-2180].
Cross-linking of the high-affinity immunoglobulin (Ig) E receptors of sensitized mast cells leads to a release of proinflammatory mediators, including a number of vasoactive cytokines resulting in acute allergic, or immediate (type I) hypersensitivity reactions [Gordon et al., Nature 1990, 346, 274-276 and Galli, N. Engl. J. Med. 1993, 328, 257-265]. A crucial role for JAK3 in IgE receptor-mediated mast cell responses in vitro and in vivo has been established [Malaviya et al., Biochem. Biophys. Res. Commun. 1999, 257, 807-813]. In addition, the prevention of type I hypersensitivity reactions, including anaphylaxis, mediated by mast cell-activation through inhibition of JAK3 has also been reported [Malaviya et al., J. Biol. Chem. 1999 274, 27028-27038]. Targeting mast cells with JAK3 inhibitors modulated mast cell degranulation in vitro and prevented IgE receptor/antigen-mediated anaphylactic reactions in vivo.
A recent study described the successful targeting of JAK3 for immunosuppression and allograft acceptance. The study demonstrated a dose-dependent survival of Buffalo heart allograft in Wistar Furth recipients upon administration of inhibitors of JAK3 indicating the possibility of regulating unwanted immune responses in graft versus host disease [Kirken, Transpl. Proc. 2001, 33, 3268-3270].
IL-4-mediated STAT-phosphorylation has been implicated as the mechanism involved in early and late stages of rheumatoid arthritis (RA). Up-regulation of proinflammatory cytokines in RA synovium and synovial fluid is a characteristic of the disease. It has been demostrated that IL-4-mediated activation of IL-4/STAT pathway is mediated through the Janus Kinases (JAK 1 & 3) and that IL-4-associated JAK kinases are expressed in the RA synovium [Muller-Ladner et al., J. Immunol. 2000, 164, 3894-3901].
Familial amyotrophic lateral sclerosis (FALS) is a fatal neurodegenerative disorder affecting about 10% of ALS patients. The survival rates of FALS mice were increased upon treatment with a JAK3 specific inhibitor. This suggested that JAK3 plays a role in FALS [Trieu et al., Biochem. Biophys. Res. Commun. 2000, 267, 22-25].
Signal transducer and activator of transcription (STAT) proteins are activated by, among others, the JAK family kinases. Results from a recent study suggested the possibility of intervention in the JAK/STAT signaling pathway by targeting JAK family kinases with specific inhibitors for the treatment of leukemia [Sudbeck et al., Clin. Cancer Res. 1999, 5, 1569-1582]. JAK3 specific compounds were shown to inhibit the clonogenic growth of JAK3-expressing cell lines DAUDI, RAMOS, LC1-19, NALM-6, MOLT-3 and HL-60.
In animal models, TEL/JAK2 fusion proteins have induced myeloproliferative disorders and in hematopoietic cell lines, and introduction of TEL/JAK2 resulted in activation of STAT1, STAT3, STAT5, and cytokine-independent growth [Schwaller et al., EMBO J. 1998, 17, 5321-5333].
Inhibition of JAK3 and TYK2 abrogated tyrosine phosphorylation of STAT3, and inhibited cell growth of mycosis fungoides, a form of cutaneous T-cell lymphoma. These results implicated JAK family kinases in the constitutively activated JAK/STAT pathway that is present in mycosis fungoides [Nielsen et al., Proc. Nat. Acad. Sci. U.S.A. 1997, 94, 6764-6769]. Similarly, STAT3, STAT5, JAK1 and JAK2 were demonstrated to be constitutively activated in mouse T-cell lymphoma characterized initially by LCK over-expression, thus further implicating the JAK/STAT pathway in abnormal cell growth [Yu et al., J. Immunol. 1997, 159, 5206-5210]. In addition, IL-6-mediated STAT3 activation was blocked by an inhibitor of JAK, leading to sensitization of myeloma cells to apoptosis [Catlett-Falcone et al., Immunity 1999, 10, 105-115].
Another kinase family of particular interest is the Src family of kinases. These kinases are implicated in cancer, immune system dysfunction and bone remodeling diseases. For general reviews, see Thomas and Brugge, Annu. Rev. Cell Dev. Biol. 1997, 13, 513; Lawrence and Niu, Pharmacol. Ther. 1998, 77, 81; Tatosyan and Mizenina, Biochemistry (Moscow) 2000, 65, 49-58; Boschelli et al., Drugs of the Future 2000, 25(7), 717.
Members of the Src family include the following eight kinases in mammals: Src, Fyn, Yes, Fgr, Lyn, Hck, Lck, and Blk. These are nonreceptor protein kinases that range in molecular mass from 52 to 62 kD. All are characterized by a common structural organization that is comprised of six distinct functional domains: Src homology domain 4 (SH4), a unique domain, SH3 domain, SH2 domain, a catalytic domain (SH1), and a C-terminal regulatory region. Tatosyan et al. Biochemistry (Moscow) 2000, 65, 49-58.
Based on published studies, Src kinases are considered as potential therapeutic targets for various human diseases. Mice that are deficient in Src develop osteopetrosis, or bone build-up, because of depressed bone resorption by osteoclasts. This suggests that osteoporosis resulting from abnormally high bone resorption can be treated by inhibiting Src. Soriano et al., Cell 1992, 69, 551 and Soriano et al., Cell 1991, 64, 693.
Suppression of arthritic bone destruction has been achieved by the overexpression of CSK in rheumatoid synoviocytes and osteoclasts. Takayanagi et al., J. Clin. Invest. 1999, 104, 137. CSK, or C-terminal Src kinase, phosphorylates and thereby inhibits Src catalytic activity. This implies that Src inhibition may prevent joint destruction that is characteristic in patients suffering from rheumatoid arthritis. Boschelli et al., Drugs of the Future 2000, 25(7), 717.
Src also plays a role in the replication of hepatitis B virus. The virally encoded transcription factor HBx activates Src in a step required for propagation of the virus. Klein et al., EMBO J. 1999, 18, 5019, and Klein et al., Mol. Cell. Biol. 1997, 17, 6427.
A number of studies have linked Src expression to cancers such as colon, breast, hepatic and pancreatic cancer, certain B-cell leukemias and lymphomas. Talamonti et al., J. Clin. Invest. 1993, 91, 53; Lutz et al., Biochem. Biophys. Res. 1998 243, 503; Rosen et al., J. Biol. Chem. 1986, 261, 13754; Bolen et al., Proc. Natl. Acad. Sci. USA 1987, 84, 2251; Masaki et al., Hepatology 1998, 27, 1257; Biscardi et al., Adv. Cancer Res. 1999, 76, 61; Lynch et al., Leukemia 1993, 7, 1416. Furthermore, antisense Src expressed in ovarian and colon tumor cells has been shown to inhibit tumor growth. Wiener et al., Clin. Cancer Res., 1999, 5, 2164; Staley et al., Cell Growth Diff. 1997, 8, 269.
Other Src family kinases are also potential therapeutic targets. Lck plays a role in T-cell signaling. Mice that lack the Lck gene have a poor ability to develop thymocytes. The function of Lck as a positive activator of T-cell signaling suggests that Lck inhibitors may be useful for treating autoimmune disease such as rheumatoid arthritis. Molina et al., Nature, 1992, 357, 161. Hck, Fgr and Lyn have been identified as important mediators of integrin signaling in myeloid leukocytes. Lowell et al., J. Leukoc. Biol., 1999, 65, 313. Inhibition of these kinase mediators may therefore be useful for treating inflammation. Boschelli et al., Drugs of the Future 2000, 25(7), 717.
Syk is a tyrosine kinase that plays a critical role in FcεRI mediated mast cell degranulation and eosinophil activation. Accordingly, Syk kinase is implicated in various allergic disorders, in particular asthma. It has been shown that Syk binds to the phosphorylated gamma chain of the FcεRI receptor via N-terminal SH2 domains and is essential for downstream signaling [Taylor et al., Mol. Cell. Biol. 1995, 15, 4149].
Inhibition of eosinophil apoptosis has been proposed as a key mechanism for the development of blood and tissue eosinophilia in asthma. IL-5 and GM-CSF are upregulated in asthma and are proposed to cause blood and tissue eosinophilia by inhibition of eosinophil apoptosis. Inhibition of eosinophil apoptosis has been proposed as a key mechanism for the development of blood and tissue eosinophilia in asthma. It has been reported that Syk kinase is required for the prevention of eosinophil apoptosis by cytokines (using antisense) [Yousefi et al., J. Exp. Med. 1996, 183, 1407].
The role of Syk in FcγR dependent and independent response in bone marrow derived macrophages has been determined by using irradiated mouse chimeras reconstituted with fetal liver cells from Syk −/− embryos. Syk deficient macrophages were defective in phagocytosis induced by FcγR but showed normal phagocytosis in response to complement [Kiefer et al., Mol. Cell. Biol. 1998, 18, 4209]. It has also been reported that aerosolized Syk antisense suppresses Syk expression and mediator release from macrophages [Stenton et al., J. Immunology 2000, 164, 3790].
The Tec family of non-receptor tyrosine kinases plays a central role in signalling through antigen-receptors such as the TCR, BCR and Fcε receptors (reviewed in Miller A, et al. Current Opinion in Immunology 14;331-340 (2002). Tec family kinases are essential for T cell activation. Three members of the Tec family, Itk, Rlk and Tec, are activated downstream of antigen receptor engagement in T cells and transmit signals to downstream effectors, including PLC-γ. Deletion of Itk in mice results in reduced T cell receptor (TCR)-induced proliferation and secretion of the cytokines IL-2, IL-4, IL-5, IL-10 and IFN-γ (Schaeffer et al, Science 284; 638-641 (1999)), Fowell et al, Immunity 11;399-409 (1999), Schaeffer et al. Nature Immunology 2,12; 1183-1188 (2001))). The immunological symptoms of allergic asthma are attenuated in Itk−/− mice. Lung inflammation, eosinophil infiltration and mucous production are drastically reduced in Itk−/− mice in response to challenge with the allergen OVA (Mueller et al, Journal of Immunology 170: 5056-5063 (2003)). Itk has also been implicated in atopic dermatitis. This gene has been reported to be more highly expressed in peripheral blood T cells from patients with moderate and/or severe atopic dermatitis than in controls or patients with mild atopic dermatitis (Matsumoto et al, International archives of Allergy and Immunology 129; 327-340 (2002)).
Splenocytes from Rlk−/− mice secrete half the IL-2 produced by wild type animals in response to TCR engagement (Schaeffer et al, Science 284; 638-641 (1999)), while combined deletion of Itk and Rlk in mice leads to a profound inhibition of TCR-induced responses including proliferation and production of the cytokines IL-2, IL-4, IL-5 and IFN-γ (Schaeffer et al. Nature Immunology 2,12; 1183-1188 (2001)), Schaeffer et al, Science 284; 638-641 (1999)). Intracellular signalling following TCR engagement is effected in Itk/Rlk deficient T cells; inositol triphosphate production, calcium mobilization, MAP kinase activation, and activation of the transcription factors NFAT and AP-1 are all reduced (Schaeffer et al, Science 284; 638-641 (1999), Schaeffer et al. Nature Immunology 2,12; 1183-1188 (2001)).
Tec family kinases are also essential for B cell development and activation. Patients with mutations in Btk have a profound block in B cell development, resulting in the almost complete absence of B lymphocytes and plasma cells, severely reduced Ig levels and a profound inhibition of humoral response to recall antigens (reviewed in Vihinen et al. Frontiers in Bioscience 5:d917-928). Mice deficient in Btk also have a reduced number of peripheral B cells and greatly decreased levels of IgM and IgG3. Btk deletion in mice has a profound effect on B cell proliferation induced by anti-IgM, and inhibits immune responses to thymus-independent type II antigens (Ellmeier et al, J Exp Med 192:1611-1623 (2000)).
Tec kinases also play a role in mast cell activation through the high-affinity IgE receptor (FcεRI). Itk and Btk are expressed in mast cells and are activated by FcεRI cross-linking (Kawakami et al, Journal of Immunology; 3556-3562 (1995)). Btk deficient murine mast cells have reduced degranulation and decreased production of proinflammatory cytokines following FcεRI cross-linking (Kawakami et al. Journal of leukocyte biology 65:286-290). Btk deficiency also results in a decrease of macrophage effector functions (Mukhopadhyay et al, Journal of Immunology; 168, 2914-2921 (2002)).
Accordingly, there is a great need to develop compounds useful as inhibitors of protein kinases. In particular, it would be desirable to develop compounds that are useful as inhibitors of GSK-3, SYK, Aurora-2, CDK-2, JAK-3, LCK, SRC, and Tec family (e.g., Tec, Btk, Itk/Emt/Tsk, Bmx, Txk/Rlk) protein kinases, particularly given the inadequate treatments currently available for the majority of the disorders implicated in their activation.